Conventional anti-inflammatory steroids, such as cortisone, hydrocortisone, prednisone, methylprednisolone, etc., are generally poorly water soluble and therefore not suited for intravenous administration. Several types of soluble C-21 derivatives of such steroids have been disclosed in the patent literature including dicarboxylic acid hemiesters, sulfobenzoates, sulfopropionates, sulfates, phosphates, and aminoalkanoyloxy derivatives. While solubilization can generally be attained quite readily using a variety of such pro-moieties, most of the aforementioned derivatives possess other disadvantages limiting their utility as water soluble prodrugs. The term "prodrug" denotes a derivative of an active drug which is converted after administration back to the active drug. The "pro-moiety" referred to in this application is the fragment attached to the steroid via an ester linkage and removed by ester hydrolysis in vivo. A major problem with many common derivatives is their solution instability. Dicarboxylic acid hemiesters of corticosteroids such as succinate esters, for example, are marketed commercially as lyophilized powders for reconstitution prior to injection due to their solution instability (see, for example, E. R. Garrett, J. Pharm. Sci., 51, 445 (1962); B. D. Anderson and V. Taphouse, J. Pharm. Sci., 70, 181; R. Yamamoto, S. Fujisawa, and M. Kawamura, Yakugaku Zasshi, 91, 855 (1971)).
Corticosteroid 21-aminoalkyl carboxylate derivatives reported in the literature also undergo rapid hydrolysis in aqueous solution (M. Kawamura, R. Yamamoto, and S. Fujisawa, Yakugaku Zasshi, 91, 863 (1971)).
Certain derivatives which do appear to exhibit better solution stability suffer from other disadvantages. 21-sulfate esters, for example, may not be readily converted to the active parent drug in vivo as suggested by the fact that the 21-sulfate of hydrocortisone is inactive in mice (M. Kawamura, R. Yamamoto, and S. Fujisawa, Yakugaku Zasshi, 91, 871 (1971); meta-sulfobenzoate esters which have been reported as having improved solution stability (M. Kawamura, R. Yamamoto and S. Fujisawa, ibid, French Patent Derwent No. 76199 U)) are frequently not highly water soluble and thus may have limited utility as injectable prodrugs. Phosphate esters may in some cases possess the requisite solubility, solution stability, and bioconversion rates but exhibit other disadvantages. Several undesirable features of phosphate esters are apparent: (1) Phosphate esters are often difficult to purify and are frequently very hygroscopic. (2) The stability of phosphate esters is optimum above pH 7 where other modes of drug degradation may be a problem. Glass surfaces are also more likely to delaminate in alkaline conditions resulting in particulate problems. (3) Precipitation of free corticosteroid due to the limited amount of hydrolysis which does occur may limit product shelf-life. Solubilization of free corticosteroid due to micelle formation by the intact prodrug is a desirable feature which phosphate esters exhibit to only a limited extent. (4) Concentrated solutions of phosphate esters of corticosteroids exhibit accelerated reaction velocities due to micelle formation, limiting shelf-life in concentrated solutions (G. L. Flynn and D. J. Lamb, J. Pharm. Sci., 1433 (1970)). Sulfopropionate esters of corticosteroids have also been reported as readily water soluble and having improved solution stability (Derwent Accession No. 27789C). Sulfoacetate esters are also known (Derwent 9453F). The esters claimed in the present invention are significantly more stable than sulfoacetate or sulfopropionate esters.